Caramelized onion potatoes au gratin is the perfect potato side dish for your Thanksgiving table. You can say goodbye to watery potato recipes this year because this recipe is fool-proof!
The caramelized onions add a little extra special something and the potatoes are perfectly creamy and cheesy. You can even make this recipe ahead of time for ease!
Table of contents
Potatoes au Gratin vs Scalloped Potatoes
While potatoes au gratin and scalloped potatoes are similar, the biggest difference is the sauce. Scalloped potatoes are cooked in a creamy sauce that begins with flour and butter (roux); whereas the sauce for potatoes au gratin is thickened by the starch in the potatoes.
There is some debate to say that scalloped potatoes don’t contain cheese, but I think both potato dishes benefit greatly from cheese. I often see cheese in both. While both are delicious, I find that scalloped potatoes curdle more readily than potatoes au gratin.
Helpful Cooking Tools
Ingredients you’ll need
- Onions. Opt for yellow onions as they have a bit more sugar and caramelize more easily.
- Potatoes. Normally I’m a Yukon gold gal but you need a starchy potato. Russet is best here.
- Aromatics. Garlic, bay leaves, nutmeg, and fresh thyme give these caramelized onion potatoes au gratin all the flavor.
- Dairy. I use a mix of heavy cream and half & half here for the perfect, silky texture.
- Cheese. Gruyere is my favorite cheese for making potatoes au gratin. You wait to aim for a semi-hard cheese. Gouda would be a great option too.
An Overview of the Cooking Process
- Caramelize the onions. It takes a while, but it’s very passive work! You don’t need to stir the entire time, just every now and then. Everything is cooked over low heat, so very little risk of anything burning. It’s the perfect thing to make while you’re multitasking with other things, just check in on it here and there. The key to caramelized onions is to make sure all the onions are the same size so I recommend using a mandoline here too.
Jenny’s Tip: I recommend making the caramelized onions ahead of time (up to 3 days!). I found that, all in all, in cooking the onions down, the whole process took about 45 minutes. Getting that done ahead of time makes preparing the potatoes au gratin that much easier!
On using a Mandoline: I highly recommend using a mandoline to slice your potatoes. Not only does it make the work go much more quickly, but it ensures that all the potatoes are the same thickness so that they cook evenly.
- Infuse the cream and cook the potatoes. Letting the potatoes partially cook in the cream mixture adds some of their starch so that the sauce is nice and thick. It also gets a jump start on the cooking so the potatoes don’t have to be in the oven as long.
- Layer and bake. The key to getting the caramelized onions and cheese throughout the gratin is to layer the potatoes, onions, and cheese in multiple layers. Every bite will have cheesy & oniony potato goodness! Bake until the cheese is browned and the potatoes are bubbling and you’re ready to go!
The Best Potato to Use when Making Potatoes au Gratin
Using the right potatoes is essential to this recipe! Because there are no thickeners in the sauce like flour, you really need a starchy potato to make everything come together.
I always use russet potatoes for this recipe as it yields the best results. I’m team Yukon gold for mashed potatoes all the way, but waxy potatoes will only leave you with a broken and soupy potato dish here. Russets all the way!
Tips for Making a Perfect Potato Gratin
- Use a mandoline. Evenly sliced potatoes cook evenly, so unless you are a whiz with a knife, use a mandoline for precise cuts.
- Simmer your potatoes. Partially cooking the potatoes in the cream sauce before baking not only speeds up the cooking time but really thickens the sauce from the starch in the potatoes. You’ll never have a broken, watery sauce again!
- Don’t use pre-shredded cheese. Shredded cheese is coated in cellulose to keep it from clumping and doesn’t melt as well. Bust out the box grater and give it a little elbow grease to shred your cheese by hand.
- Use the right cheese. You want a semi-hard cheese with lots of flavor. Gruyere, gouda, and Emmental would all be great choices.
- Let the potatoes rest. After removing them from the oven, let your potatoes rest for at least 15 minutes before serving.
Easy Make Ahead Instructions
I love that this recipe can be made ahead of time. It can save you so much time and stress during the holidays.
- Follow the recipe up until the potatoes au gratin go in the oven. Layer them in the baking dish with the caramelized onions and cheese, cover with foil, and refrigerate overnight.
- Remove them from the fridge 1 hour before baking. Bake covered for 20 minutes and uncovered for an additional 20-25 minutes until the top is golden and the cheese is bubbly.
If you do give this recipe a try, I love hearing about it! Leave a comment with a star rating below. Be sure to subscribe to my weekly newsletter and never miss a new recipe! You can also snap a photo & tag @JENNYGOYCOCHEA on Instagram.
More Delicious Potato Side Dish Recipes
Sour Cream & Onion Mashed Potatoes
Hasselback Potato Gratin
Loaded Twice Baked Potatoes
Ham & Potato Soup
Roasted Garlic Mashed Potatoes
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Caramelized Onion Potatoes au Gratin
Video
Ingredients
- 4 tablespoons unsalted butter, divided
- 2 sweet yellow onions, thinly sliced
- 1 teaspoon sugar
- 4 lbs (about 4-7) large russet potatoes, peeled and sliced about 1/8" thick (3mm) on a mandoline
- 2 1/2 cups heavy cream
- 1 1/2 cups half & half
- 3 garlic cloves, grated
- 2-3 sprigs of fresh thyme
- 2 bay leaves
- Pinch of Nutmeg
- 6 oz gruyere cheese, shredded
- Kosher salt
- Freshly ground pepper
Instructions
- In a medium skillet, melt 2 tablespoons of butter over medium-low heat. Add the onions to the skillet and toss to coat. Cover the pan for 5 minutes to help the onions wilt. Season with some salt and pepper and cook, stirring occasionally, until they start to turn golden brown. Sprinkle with the sugar and cook until deeply amber in color. Set aside to cool.
- In a 4 qt pot or dutch oven, combine the cream, half and half, garlic, thyme, bay leaves, and nutmeg. Season with salt and pepper. Add the potatoes to the pot and bring to a gentle simmer over low heat. Simmer for 15 minutes, or until the potatoes are just starting to become tender. Season to taste with salt and pepper, if needed. Remove from the heat and rest for 10 minutes.
- Preheat an oven to 375 F. Butter a 3 qt baking dish with the remaining butter and set aside. Discard the thyme sprigs and bay leaves from the potatoes.
- Using a slotted spoon, transfer half of the potatoes to the prepared baking dish. Layer in half of the caramelized onions and cheese and cover with the rest of the potatoes. Pour the remaining creamy sauce in the pot over the potatoes and top with the remaining onions and cheese. Bake uncovered for 35-40 minutes until the cheese is bubbly and golden brown and the potatoes are tender.
- Rest the potatoes for 15 minutes before serving. Top with more fresh thyme, if desired.
Rate & Review This Recipe
[…] 20. Caramelized Onion and Potato Gratin […]
I am nearly through the prep and am seeing that my mandolin is much thinner than the recommended thickness. Wish me luck
If it’s a little thin, that’s okay! Just cook the potatoes less and it should all still be fine.
Hi! Can I make any part of this a day ahead of time?
Yep! Info on make ahead instructions is in the blog post.
Would this be easy enough to double this recipe? Potentially just bake a little longer since a bigger pan will obviously be necessary?
Yes! Should be fine.
Flavors are likely good but the instructions could use improvement. Low simmer should be low boil. Minimum of 5 or 6 quart pot so it’s not overflowing and larger baking dish as well. Without a ton of work this recipe could be simplified.
Wrong. You really should not be boiling cream or milk as it will curdle and hurt the texture of the gratin. This recipe has been tested exhaustively and the instructions are the way they are for a reason. The Dutch oven used in the recipe and the photos is an 4 qt, but you could use a larger one if that’s what you have. Also, a 3 qt baking dish is considered a large baking dish. Perhaps just consider that sometimes good food takes time and effort and that’s okay!
Made this for Thanksgiving. The only change (could not help myself) was pre-roasting the garlic with evoo in foil at 375 until savory and soft. Oh, and I mixed in some Comte with the Gruyère for a fun kick. These potatoes were heaven on a fork. The kind of bite where you stop talking, close your eyes and silently savor the beauty of life for a floating moment of bliss. Thank you! PS I had to bake longer than the instructions. Totally expected for variations of potato thickness, oven differences, etc. Bookmarked this recipe for future holidays <3
Love that idea of the roasted garlic! Thank you for sharing your experience and I’m so happy you loved this recipe as much as I do!